1. Field of the Invention
The present invention relates to an ink jet recording apparatus, an ink supplying mechanism, and an ink supplying method for ejecting an ink from an ink jet head while circulating the ink.
2. Description of the Related Art
A technique for ejecting an ink from a nozzle of an ink jet head while circulating the ink in an ink jet recording apparatus is disclosed in, for example, JP T 2002-533247 (the term “JP-T” as used herein means a published Japanese translation of a PCT patent application) or US 2002/0118256A1. In such an ink jet recording apparatus, for example, an upstream side tank, an ink jet head, and a downstream side tank are connected by a conduit. A liquid surface of the upstream side tank and a liquid surface of the downstream side tank are kept constant. An ink in the upstream side tank circulates to flow into the ink jet head through an upstream side channel and flow into the downstream side tank through a downstream side channel.
In such an ink jet recording apparatus, to prevent deficiencies such as inclusion of air and ink leakage and secure a satisfactory printing characteristic, maintenance of a proper circulation flow rate is demanded. In the technique described above, a circulation flow rate depends on a channel resistance of a channel extending from the upstream side tank to the downstream side tank via the upstream side channel, the ink jet head, and the downstream side channel and a difference between the height of the upstream side tank and the height of the downstream side tank. Therefore, in order to adjust the flow rate, it is necessary to adjust the flow rate according to positions of the upstream side tank, the downstream side tank, the ink jet head, and the like. In other words, for example, in order to increase the flow rate, it is necessary to increase the difference between the height of the upstream side tank and the height of the downstream side tank. Thus, the upstream side tank has to be lifted and the downstream side tank has to be lowered. However, usually, since an arrangement of tanks is often physically limited, it is difficult to adjust the heights. Further, since the channel resistance changes according to the change of the difference between the heights, it is difficult to secure a desired flow rate.
On the other hand, in the ink jet head, in order to secure the satisfactory printing characteristic, an ink pressure near the nozzle is extremely important. It is necessary to keep the ink pressure near the nozzle in a proper range. However, in the technique described above, when there is no ejection of the ink or an ejection quantity of the ink is small, the ink pressure near the nozzle depends on a channel resistance of a channel extending from the upstream side tank to the nozzle in the ink jet head via the upstream side channel, a channel resistance of a channel extending from the nozzle in the ink jet head to the downstream side tank via the downstream side channel, and the heights of the liquid surfaces of the upstream side tank and the downstream side tank. Therefore, in order to obtain an ink pressure in an appropriate nozzle position, it is necessary to adjust the height of the upstream side tank and the height of the downstream side tank. Consequently, the physical limitation on the arrangement of tanks and the change of channel lengths make it difficult to adjust the heights.